Chapter 13 - Main Spar - The wings have begun!

The wings have officially begun!

My intention was to complete the empennage fairings before beginning on the wing, but i figured that these were basically 2 mutually exclusive work areas and i could do both at the same time... plus the wings were calling to me!

Lets drill those spars! 

OMG! The very first step in the wing plans was to cut some J stiffners, then match drill them using the holes in the flanges of the spar. I read a lot on this step (don't know why, it was straight forward), and lots of blogs recommended cutting the Wing J Stiffners from the 8' long lengths and leaving the 6' lengths for the leading edges and the tanks. I read ahead in the plans and did the maths and concurred with this idea:

Once i was happy, i measured the J stiffners as per the plans and cut them with the Dremel, working up to the line i marked using the scotchbrite wheel. They were then clamped to the spar flanges and match drilled #40. 

The green tape marks the lower access panel areas where you do not match drill the J Stiffners. There is a sneaky hole either side of the access panel nutplates which should not be match drilled. 

Holy Countersink Batman!

Countersinking Round 1 - holes flush with the surface.

The plans called for countersinking the nutplate attach holes first - these were done with the countersink cage and and were for a flush rivet. The plans don't explicitly state this, but where the ribs attach to the spar, in the location where the fuel tank is, the forward most hole of the top and bottom tabs (were they nest inside the spar flanges) are installed with a flush rivet. This makes sense if you think about it - the tank is removable with screws, so the rib attach at these locations should be countersunk for a flush rivet (not for a skin dimple). The same holes for the ribs where the outboard leading edge is located are countersunk for a dimple (the rivet here goes through the leading edge skin, through the spar flange and finally through the rib tab). 

In order not to confuse which holes are which, i marked the 'flush to the surface' holes with a marker ready for round 1 countersinking. This was done with nothing backing up the countersink pilot. 

The middle hole with no nutplate is one of the rib locations.

The hole is countersunk so the rivets are flush with the surface. 

NOTE: There are 4 nutplates which i countersunk at the ends of the sparts on the spar web. I didn't need to do these as they are for mounting the outdated halogen landing light driver boxes - I have countersunk the holes so will install the nutplates, but for anyone after me, i would leave these off.

Countersinkng Round 2 - Fuel Tank Screw Holes

The method called for in the plans involves countersinking round 1 as above, then installing the nutplates. The idea is that the nutplates give something for the countersink pilot to 'run in' to prevent chatter and get a nice round countersink. These nutplates are K1100-08 - the 1100 signifies that the centre hole has a large dimple in it. In this case, these are called for because as you will see soon, these countersinks result in a knife edge to the hole - if you used normal K1000-08 nutplates, then the cutter could interfere with the nutplate. If however you were to use a jig like i have done below, these countersinks are not needed anymore and normal K1000-08 would be OK (but too late now, i will just use the supplied ones). 

The plans called for the spar to be primed, as the anodising will be removed once you do all this countersinking - this would have meant one round of priming to prime the nutplate attach holes done in Round 1, install the nutplates, then another round (with the nutplates in place) once the rest of the countersinking was done. I was not a fan of this idea, so decided to make up a small nutplate install jig out of 0.060 thick angle, to provide something for the countersink cutter pilot to run in. This worked exceptionally well for me and the hole in the jig showed very little signs of wearing (boelube was used on the cutter pilot and faces). 

A test piece of 0.032" skin was made up (same thickness as the fuel tank skin), with both the #8 screw and #6 screw dimples to test how the countersinks were looking. For EVERY hole i checked i was under the maximum inner and outer diameters with a caliper and to make sure the countersink cage was not wandering.

Test coupon out of 0.032" scrap. 

The nutplate jig - the backside was countersink roughly to fit AD3-3.0 rivets which you can see poking out. These align the jig to the prepunched nutplate attach holes.

The rivets above were held in place with a piece of scrap and some tape. 

 The jig was held in place with a couple of spring clamps..

Under the 0.370 max outer diameter..

... and under the 0.220 Max Inner Diameter

Because i did not install the nutplates yet, each hole was very gently deburred from the back side with a deburring tool.

Lots of "Fairy Dust" aka the payload enhancement program

A test coupon installed to see how the fuel tank skin will nest into the dimple. 

For the 'one sided' nutplates, a modified the same countersinking jig with a set of holes to allow for left and right oriented nutplates.

The jig being held in place - you can see how it supports the pilot of the countersink cutter. 

Countersinking Round 3 - the access panel attach screw holes.
These were all done in exactly the same manner as the fuel tank screw holes, except i adjusted the cage to meet the new dimensions in the plan, and they were tested against the #6 dimple on the test coupon.

Countersinking Round 4 - the skin holes
Once all of this was done, next came the marathon effort to finish off all the other holes on the spar. The remaining holes need to be countersunk deeper than flush to take a dimpled skin. It should be noted that the outboard skins are 0.025" which while the inner skins are 0.032", so initially i thought i needed 2 different countersink depths... which technically is probably true. However, section 5 of the manual simply asks for all countersinks which will accept a skin to be done 0.007" deeper than flush (irrelevant of the skin thickness).

During the countersinking on the practice kits i had the same query (and asked Van's about it).

So in the end i just did all the above holes to 0.007" deeper than flush, and used the cutter with nothing behind it to back it up. I found the best method was to start at perhaps half drill speed (I used my electric Ryobi drill), with light pressure to get it started, then full speed and higher pressure at the end. This seemed to work well and no holes were elongated.

SO... MANY... COUNTERSINKS!